(1) Field of the Invention
The present invention relates to a tail assembly for a rotorcraft, a rotorcraft and a method to manufacture a strengthened tail assembly. More particularly, the invention relates to a tail assembly for a helicopter.
Thus, the invention belongs to the technical field of rotorcraft tail assemblies.
(2) Description of Related Art
A rotorcraft has at least one main rotor mechanically driven by at least one engine. Such main rotor provides the rotorcraft with lift and possibly with propulsion.
The main rotor is carried by a cell including a tail assembly. This tail assembly comprises a tail boom, the tail boom carrying a vertical aerodynamic stabilizer which is also named “tail fin” or “fin” by the one skilled in the art. The tail assembly can also be provided with at least one aerodynamic stabilizer arranged horizontally and carried by the tail boom or by the tail fin.
The tail assembly of a rotorcraft is also provided with a tail rotor that performs an anti-torque function so as to compensate for the yaw torque created by the rotation of the main rotor, the tail rotor exerting thrust transversely. Furthermore, the tail rotor enables the pilot of the rotorcraft to control yaw and command steering movements of the rotorcraft by exerting positive or negative transverse thrust.
The tail rotor can be a non-ducted tail rotor referred to as a “conventional” tail rotor, for convenience. Conventionally, the non-ducted tail rotor is mounted on one lateral side near a top end of the tail fin or on one end of the tail boom of the rotorcraft.
Such a non-ducted tail rotor is in widespread use. Nevertheless, a ducted tail rotor can also be implemented, such ducted tail rotors being commonly known under the trademark Fenestron®.
A ducted tail rotor comprises a rotor arranged in a passage formed through the vertical tail fin of a helicopter, the axis of symmetry of the passage being substantially perpendicular to the vertical anteroposterior plane of symmetry of the rotorcraft. As a result, the streamlined structure of the tail fin surrounds said passage and thus the tail rotor. Consequently, the wall of the passage itself is also known to the person skilled in the art as a “duct” or “shroud”, which explains why it is referred to as a “ducted tail rotor” or a “shrouded tail rotor”.
Moreover, rotorcrafts require a lightweight structure to maximize the useful load that can be carried. This goal is achieved by the use of lightweight materials with high strength per weight and a structure designed so as to provide optimized load paths and stress distribution.
Nevertheless, the interface zone between the tail fin and the tail-boom is an area where such a strength and light weight design is difficult to achieve. This is even harder to achieve when the tail rotor is ducted and arranged in this tail fin.
Indeed, a tail boom is provided with a cross section made as small as possible to limit the aerodynamic interaction with the flow of air coming from the main rotor. The cross section of the tail boom is additionally constraint by the required clearances to the main rotor, flare maneuvers and rear access to the helicopter.
Conversely, a tail fin and in particular a tail fin provided with a ducted tail rotor is provided with a larger section. The cross section of the tail fin of a ducted tail rotor is made larger than the cross section of the tail boom in order to be able to host the tail rotor, both in transverse and mostly upwards directions. The tail fin as a vertical aerodynamic surface equally requires a long and thin cross section for involving an optimum effect.
So, the tail fin which presents large cross-section interfaces in the interface zone with the tail boom which conversely has a small cross-section at a sharp corner. The sudden change of cross-sections at a sharp corner between the tail boom and the tail fin is known to result in stress concentrations in the structure in the region of the interface zone. The interface zone represents the highest stress concentration zone in the tail assembly.
Moreover, the rotorcraft is provided with a power transmission shaft which links the tail rotor to a power plant to drive in rotation said tail rotor.
The power transmission shaft can be arranged above the tail boom for maintenance purpose. Indeed, if the power transmission shaft was arranged in the tail boom, the access to this power transmission shaft would be difficult.
So the power transmission shaft is often extending longitudinally along and above the tail boom.
According to the document DE202012002493 for example, the leading edge of the tail fin is consequently cut-out in the area of the interface zone to provide an aperture in the tail fin. The power transmission shaft then penetrates into the tail fin through the cut-out of the leading edge of the tail fin to reach the tail rotor.
Such cut-out for the tail rotor power shaft is located right beside or in the interface zone, further increasing the stress concentration in this interface zone.
Consequently, the interface zone can require to be strengthened to be sufficiently stable. In this situation, the interface zone can require stronger sizing of its structure and is therefore weight intensive.
It would therefore be desirable to design an alternative tail assembly.
In order to avoid a high stress concentration in the interface zone, the tail boom diameter can be increased to reduce the stress concentration in this interface zone. Nevertheless, an increase of the tail-boom dimensions upwards would decrease the clearance to the main-rotor introducing weight penalties in form of a longer rotor mast with corresponding higher bending loads. An increase of the tail boom dimensions downwards would decrease the space below the tail boom needed for flare clearance or loading. Consequently, such solutions can lead to a weight increase.
The installation of the power transmission shaft inside the tail boom can increase the interface cross-section between the tail fin and the tail boom, by the utilization of the height used conventionally for the power transmission shaft above the tail boom. Nevertheless, this solution limits accessibility to the power transmission shaft and therefore complicates maintenance and installation process.
The documents CA2821443, RU2206475, U.S. Pat. No. 5,209,430, U.S. Pat. No. 5,108,044 and U.S. Pat. No. 4,708,305 are also known but are far from the invention technical problem and domain.
The document CA2821443 describes a system and a method to control fuselage torque of an aircraft. A tail boom has a first surface that creates a high-pressure region in a downward wash by the rotor and a second surface that creates a low-pressure region in the downward wash by the rotor.
A tail rotor power shaft can extend in the tail boom.
The document RU2206475 describes a tail boom provided with three or more longitudinal planes to enhance the efficiency of tail assembly.
U.S. Pat. No. 5,209,430 describes a system to improve yaw control on a rotorcraft at low speed. This system includes strakes arranged on a tail boom.
A tail rotor power shaft can be arranged on the top of the tail boom and can be covered with a tail rotor shaft cover.
U.S. Pat. No. 4,708,305 is also showing a tail boom provided with strakes.
U.S. Pat. No. 5,108,044 shows a ducted tail rotor. The ducted tail rotor includes a shroud integral with a tail boom, a tail rotor power shaft being arranged continuously inside the tail boom and then the shroud.
Other documents are cited, i.e. U.S. Pat. No. 4,809,931, U.S. Pat. No. 4,927,331, FR2167249, U.S. Pat. No. 5,251,847 and U.S. Pat. No. 5,306,119.